The present invention relates to an antenna having a rotating phase center. The antenna may be used, for example, as a feed for a tracking antenna.
High gain antennas often employ a compound structure including a relatively small feed and a substantially larger lens or reflector for focusing electromagnetic energy at the feed. High gain antennas specifically designed for tracking moving signal sources generally utilize special antenna feeds, known as tracking feeds, for deriving signal information to be used to control movement of the antenna in such a manner that it remains pointed at the signal source. In a conical scan antenna, the main receiving element associated with the tracking feed is located slightly off of the focal point of the associated lens or reflector, and is physically rotated around the focal point. As long as the signal source is located off the boresight of the antenna, the received signal is modulated in accordance with the rotation of the main receiving element. Tracking information can be derived directly from the received signal by determining the phase and amplitude of the modulation.
Another type of tracking feed does not utilize mechanical rotation of the receiving element. Instead, four crossed dipole elements are positioned at fixed locations around the main receiving element. The crossed dipoles are designed so that they interact only mildly with the electromagnetic wave, shifting the phase center of the feed without affecting its amplitude pattern. The extent to which the crossed dipole elements shift the phase center of the feed is dependent upon the impedances of the crossed dipole elements. The impedance states of the crossed dipoles are electronically switched so as to thereby change the location of the phase center of the main receiving element. By changing the impedance states of the four crossed dipole elements in a sequential manner, the phase center of the receiving element can be rotated around the main receiving element. This imparts the same type of modulation to the received signal as does a conical scan tracking feed.
Since the phase center is rotated electronically, the feed does not require any mechanically moving parts. Unfortunately, however, the approach cannot be readily implemented at frequencies of 60 GHz or greater. The solid-state electronic components used to switch the dipole elements themselves have impedance characteristics which vary with frequency. The impedance characteristics of the solid state elements at frequencies of 60 GHz or greater are such that they cannot readily be used to provide the impedance switching function.